Many machines include on-board systems such as electronics, software, etc., that perform certain functions that make use of clock information. These functions may include, for example, logging and reporting the occurrence of events at specific times, automatically performing certain tasks at predetermined times, etc. Furthermore, a number of machines that are geographically dispersed, but yet part of the same fleet, may exchange information with each other by sending messages directly to each other. These messages may include timestamps indicating, for example, the time a message was sent from a work machine, the time a particular task was performed by a work machine, etc.
It is important to keep clock information consistent within various devices on a machine and also between physically separate machines. This may be done to ensure, among other things, accurate and efficient data communications between the physically separate machines and also between devices located on the same machine. A machine may receive clock information from various sources. In one instance, a machine may receive clock information from a clock on board the machine. Specifically, an electronic component on a machine may receive clock information from a battery-operated clock located on the machine. A battery-operated clock may include, among other things, an oscillator, a counter, and a battery.
However, there may be problems associated with obtaining clock information from an on-board battery-operated clock. Specifically, the battery-operated clock may not provide accurate clock information. This lack of accuracy may be due to the climatic conditions in which the machine operates. For example, a machine may operate in extreme climatic conditions including, for example, cold nights and hot days. A large temperature gradient may cause the battery-operated clock to lose its accuracy over a period of time. Therefore, as time goes by, some functions performed by the machine may be affected by this lack of clock accuracy. For example, messages sent by a machine including time stamps generated by an on-board battery-operated clock may not include accurate clock information. This lack of clock accuracy may lead to problems in many instances, especially where other entities, such as, for example, other machines, or an off-board control system, may rely on time-sensitive information sent by the machine.
In order to remedy the problem of inaccurate clock information being available from an on-board clock, some machines may use external clock sources, i.e., clock sources that are located off board a machine. These external clock sources may include more sophisticated and more accurate time keeping equipment. One such system is described in U.S. Pat. No. 6,535,926 B1 (“the '926 patent”) to Esker, which issued on Mar. 18, 2003. The '926 patent discloses the use of a global independent synchronization pulse to synchronize local clocks used for local events in an industrial control system. The method includes detecting at a first industrial controller, a global reference pulse and storing the time value of the master clock. This time value is transmitted by the first industrial controller as a time message on the network to other industrial controllers. A second industrial controller also detects the global reference pulse and stores a value of the local clock at that time. It then receives the time message from the first industrial controller and compares its stored local time with the time value of the first time message to deduce an error value. This error value is used to correct the clock of the second industrial controller. The '926 patent discloses that the global reference pulse may be obtained from an external clock source such as a global positioning system (“GPS”).
While the '926 patent discloses the use of an external clock source to help synchronize time across industrial controllers, it has several shortcomings. For example, all the controllers in the '926 system have to connect to the external clock source. This may be problematic because not every controller in a network may have the capability to connect to an external clock source. Furthermore, the '926 patent does not disclose the use of more than one external clock source. This may be problematic if the only external clock source being used becomes unavailable.
The present disclosure is directed to overcoming one or more of the problems of the prior art time synchronization system.